A variety of manufacturing techniques such as hot embossing, casting, and injection molding have been used to produce optical elements in the micrometer scale. Of those techniques, injection molding has been found to be preferable over the others because it allows for high production output rates with very short processing times. Conventional injection molding of optical elements, such as lenses, filters or optical waveguides involve the injection of molten optical materials (typically a polymer) into a moldplate that contains one or more cavities with predetermined optical patterns to be replicated.
After the molten optical material has been injected into the moldplate cavities, the optical material is cured and the desired optical element with a predetermined optical pattern (e.g., curvature, diameter, focal length and the like) is formed inside the cavities of the moldplate. The thus formed optical element is then extracted from the moldplate using a vacuum chuck or a similar mechanical apparatus. Removing the newly formed optical element from the moldplate cavities is often difficult, especially if the optical material remains adhered to the walls of the cavities. To prevent or minimize adhesion of the optical material to the moldplate cavities, a conformal coat of release layer is conventionally applied to the surfaces of the moldplate cavities. This release layer is generally damaged when the optical element is removed from the moldplate. Consequently, the damaged release layer is removed, and a new release layer is applied for the injection molding of a new optical element. Evidently, the structure of the moldplate cavities is a critical component in an injection molding system.
Newer micro and nanometer optical applications require optical elements with very shallow radius of curvature and very precise dimensions (e.g., micro-lenses with a radius of curvature in the order of few hundred nanometers to less than one micron are highly desirable for applications such as integration of CMOS or CCD digital cameras in mobile telephones, optical-couplers for solid-state lasers and photodetectors, launch device elements for optical fiber communications, optical fiber interconnection, optical waveguides, muxes for WDM, planar lightwave circuits, photonic devices, and solar cells for electricity generation. However, high-precision polymer optical components are very difficult to fabricate by the method of injection molding because very high temperatures are required to melt the polymer optical material for injection, and at the same time, rapid cooling of the molten material is desired for efficient mass-production. This fast change in temperature often causes damage to the cooled polymer and prevents the proper formation of an optical element with highly precise dimensions. In addition, other complications of the injection molding process are burning or scorching of parts due to melt temperature being too high or curing cycle time being too long; warping of parts due to uneven surface temperature of the moldplate cavities; surface imperfections and bubbles due to incomplete filling, surface cracking due to rapid change of temperature, and the like. It is furthermore a significant economic advantage to enable the attachment of optical elements directly to optoelectronic devices with lithographic precision at the wafer-scale or large substrate level in manufacturing of optoelectronic and optomechanical subsystems.
In order to obtain high-precision optical elements, it is thought that, in principle, high precision moldplate cavities combined with slow cooling of the injected optical material could provide the required precise dimensions. However, a high precision moldplate would result in a very costly and low re-utilization solution. Moreover, a slow cooling process would result in increased production times which may be unsuitable for efficient mass-production of optical elements.
In view of the foregoing and other considerations, there is a clear need to develop a low-cost, high-reutilization moldplate that would allow for the manufacture of very precise optical elements with rapid turnaround time.